Statements of Cash Flows - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Cash Flows from Operating Activities
Net increase (decrease) in net assets resulting from operations	$ 174,936,959	$ (190,208,450)	$ 80,418,616
Adjustments to reconcile net increase (decrease) in net assets resulting from operations to net cash provided by (used in) operating activities:
Purchases of short-term investments	(6,476,139,465)	(14,886,123,153)	(7,773,697,622)
Sales/maturities of short-term investments	6,946,868,283	15,054,622,653	7,442,051,201
Accretion of discount	(21,386,248)	(24,490,437)	(9,702,292)
Net realized (gain) loss on short-term investments	(96,367)	(3,343)	(2,812)
Net change in unrealized appreciation/depreciation on short-term investments	(38,916)	(77,730)	59,334
Change in operating assets and liabilities:
Receivable for variation margin on open futures contracts	920,643	4,637,434	(5,558,077)
Payable for variation margin on open futures contracts	5,952,581	0	(1,187,213)
Sponsor’s fees payable	(340,100)	(41,821)	206,124
Brokerage commissions and fees payable	(8,046)	2,171	(5,102)
Net cash provided by (used in) operating activities	630,669,324	(41,682,676)	(267,417,843)
Cash Flows from Financing Activities
Contributions for Shares issued	180,290,776	281,055,224	346,345,925
Distributions for Shares redeemed	(789,751,239)	(271,125,530)	(72,418,741)
Net cash provided by (used in) financing activities	(609,460,463)	9,929,694	273,927,184
Net increase (decrease) in cash and cash equivalents	21,208,861	(31,752,982)	6,509,341
Cash
Beginning of year, Unrestricted — cash	10,033,297	41,786,279	35,276,938
End of year, Unrestricted — cash	$ 31,242,158	$ 10,033,297	$ 41,786,279